introduce dedicated nr pdcp tick thread for F1 split

Currently, NR PDCP ticks are active only with monolithic gNB, triggered by the MAC scheduler in gNB_scheduler.c.
In F1 split mode, as there is no MAC, there’s no source to advance the NR PDCP timer via "nr_pdcp_wakeup_timer_thread()".
This commit introduces a dedicated thread for NR PDCP ticks in gNB-CU and CU-UP mode.
Without it, packet losses at the F1 uplink would cause indefinite buffering, as t-Reordering won't expire due to the lack of NR PDCP timer ticks.

executables/nr-uesoftmodem.c

@@ -317,7 +317,7 @@ static void init_pdcp(int ue_id)
   if (get_softmodem_params()->nsa && rlc_module_init(0) != 0) {
     LOG_I(RLC, "Problem at RLC initiation \n");
   }
-  nr_pdcp_layer_init(false);
+  nr_pdcp_layer_init();
   nr_pdcp_module_init(pdcp_initmask, ue_id);
 }